A prior-art heat exchanger constituting an air conditioner includes a heat exchanger called fin-tube heat exchanger. This heat exchanger is constituted by plate-like fins arranged with a certain interval and through which gas (air) flows and a flat-shaped heat transfer pipe inserted orthogonally into the plate-like fins and through which a refrigerant flows, and a plurality of protruding strips are provided in the axial direction on an inner face of the heat transfer pipe (See Patent Document 1, for example). Also, a heat exchanger having a flat-shaped heat transfer pipe in a multi-hole structure or a heat exchanger having a plurality of slits provided in a plate-like fin by cutting are included. The slit group is provided so that a side end portion of the slit opposes a flow direction of air, and it is described that by thinning a speed boundary layer and a temperature boundary layer of the air flow at the side end portion of the slit, heat transfer is promoted and heat exchange capacity is increased (See Patent Document 2, for example).